1. Technical Field of the Invention
The present invention relates to an EUV light generator for use in a light source such as an exposure device and, in particular, to a method and apparatus for cleaning a collector mirror for collecting EUV light.
2. Related Art
Optical lithography to optically transfer circuit patterns onto semiconductor wafers is important for integration of LSIs. Exposure devices used for the optical lithography are typically of a reduced projection exposure type, which are called steppers. Specifically, an original pattern (reticle) is irradiated with light from an illumination light source, and the transmitted light is projected on a photosensitive material on a semiconductor substrate by a reduced projection optical system to form a circuit pattern. The resolution of this projected image is limited by a wavelength of the used light source. Therefore, the wavelength of the light source has been gradually reduced into the ultraviolet region to meet the needs for further reduction of the pattern line width.
In recent years, KrF excimer lasers (with a wavelength of 248 nm) and ArF excimer lasers (with a wavelength of 193 nm) oscillating light in a deep ultraviolet region (DUV light) have been used as light sources. Further, F2 lasers (with a wavelength of 157 nm) oscillating light in a vacuum ultraviolet region (VUV light) have also been developed as light sources.
Today, attempts are being made to employ, as light sources for the optical lithography, EUV light sources (with a wavelength of 13.5 nm) outputting light in an extreme ultraviolet region (hereafter, referred to as the EUV light) for the purpose of enabling further miniaturization.
The laser-produced plasma (LPP) method is one of methods available for generating EUV light.
An EUV light source employing the LPP method applies short pulse laser light to a target to excite the target into a plasma state, and thereby generates EUV light. The generated EUV light is collected by a collector lens and output to the outside.
FIG. 1 is a conceptual diagram showing a configuration of an LPP-type EUV light generator used as a light source for an exposure device.
A collector mirror 3 for collecting EUV light is provided in the inside of a vacuum chamber 2. The EUV light collected by the collector mirror 3 is transmitted to an exposure device (not shown) outside the vacuum chamber 2. The exposure device uses this EUV light to form a semiconductor circuit pattern on a semiconductor wafer.
The inside of the vacuum chamber 2 is evacuated to form a vacuum state by a vacuum pump or the like. This is because it is only in the vacuum that the EUV light having a wavelength as short as 13.5 nm can be propagated efficiently.
The target 1 serving as an EUV light generation source is located at a predetermined EUV light generation point A within the vacuum chamber 2, that is, at the condensing point of laser light. The target 1 is made of a material such as tin (Sn), lithium (Li), or xenon (Xe).
Laser light L is pulse-oscillated in the driving laser device 4 serving as a laser oscillator and the laser light L is emitted therefrom. A Nd:YAG laser, CO2 laser or the like is used as the laser.
The laser light L is focused at the EUV light generation point A through a laser condensing optical system. The laser light L is applied to the target 1 at the timing when the target 1 is located at the EUV light generation point A. The target 1 is excited to a plasma state by the application of the laser light L to the target 1, and EUV light is generated thereby.
The generated EUV light is scattered in all directions around the plasma. The collector mirror 3 is disposed so as to surround the plasma. The collector mirror 3 collects the EUV light scattered in all directions and reflects the EUV light. The collector mirror 3 selectively reflects the light with a desired wavelength of 13.5 nm. The EUV light reflected by the collector mirror 3 (output EUV light) is transmitted to the exposure device.
A part of the target 1 is split and scattered to produce debris by shock waves during generation of the plasma. The debris includes residue of the target 1 which is left after production of fast ions or plasma.
The scattered debris adheres to the surfaces of optical elements including the collector mirror 3 within the vacuum chamber 2, specifically on the surfaces of the collector mirror 3, a laser condenser lens, a mirror, a laser light entrance window, a SPF (spectrum purity filter), and an entrance window of an optical sensor. This causes a problem of reduction of reflectance and transmittance of the optical elements, resulting in deterioration of the EUV light output, or deterioration of the sensitivity of the optical sensor.
In order to solve this problem, Japanese Patent Application Laid-open (Translation of PCT application) No. 2005-529052 proposes a technique in which ions emitted from plasma are trapped by a magnetic field and discharged out of the vacuum chamber 2. For example, when a CO2 laser is used as the driving laser device 4 for exciting a target, and a metal target of tin (Sn) is used as the target 1, most of the tin (Sn) is converted into a plasma state in which excited multi-charged positive Sn ions are separated from electrons. If a magnetic field is applied to the periphery including this target plasma, positive Sn ions are trapped in the magnetic field, whereby the movement of the positive Sn ions is limited to the direction along the magnetic field lines. Thus, the positive Sn ions can be trapped in the magnetic field and moved in a direction along the magnetic field lines to avoid the optical elements including the collector mirror 3, so that the Sn ions can be prevented from adhering to the optical elements such as the collector mirror, and the Sn ions can be efficiently discharged out of the vacuum chamber 2.
However, the multi-charged positive Sn ions thus generated are apt to be recombined with the generated electrons. Some of the recombined Sn ions are possibly neutralized and adhere as neutral debris to the optical elements including the collector mirror 3 without being trapped by the magnetic field. Additionally, it is difficult to ionize the entire target 1 by means of the driving laser device 4 for exciting the target and a part of the target 1 possibly adheres as neutral particles to the optical elements including the collector mirror 3 without being trapped by the magnetic field.
In order to solve this problem, Japanese Patent Application Laid-open (Translation of PCT application) No. 2006-529057 proposes removing the debris adhering to the collector mirror 3 with the use of a reactive gas or the like.
Among the optical elements within the vacuum chamber 2, it is the collector mirror 3 that is most likely to be contaminated with the debris adhering thereto and is most likely to require cleaning.
Ions adhering to the collector mirror 3 can in principle be removed by cleaning the collector mirror with the use of a reactive gas or the like, as described in Japanese Patent Application Laid-open (Translation of PCT application) No. 2006-529057. After the cleaning, the reflectance of the collector mirror 3 is restored and the collector mirror 3 can be used continuously.
However, the collector mirror 3 must be isolated from the vacuum chamber 2 during the cleaning of the collector mirror 3, and hence the EUV light cannot be collected with the collector mirror 3 during the cleaning process. Further, when the collector mirror 3 has come to the end of its useful life and the cleaning is not helpful anymore, the collector mirror 3 must be replaced with a new one. Again, the EUV light cannot be collected with the collector mirror 3 during the replacement of the collector mirror. Thus, the EUV light generator suffers significant downtime during the cleaning and replacement of the collector mirror.